Summary

The font-size-adjust CSS property specifies that font size should be chosen based on the height of lowercase letters rather than the height of capital letters. This is useful since the legibility of fonts, especially at small sizes, is determined more by the size of lowercase letters than by the size of capital letters.

Legibility can become an issue when the first-choice font-familyis unavailable and its replacement has a significantly different aspect ratio (the ratio of the size of lowercase letters to the size of the font).

To use this property in a way that is compatible with browsers that do not support font-size-adjust, it is specified as a number that the font-size property is multiplied by. This means the value specified for the property should generally be the aspect ratio of the first choice font. For example, a style sheet that specifies

font-size: 14px;
font-size-adjust: 0.5;

is really specifying that the lowercase letters of the font should be 7px high (0.5 × 14px).

Values

Choose the size of the font so that its lowercase letters (as determined by the x-height of the font) are the specified number times the font-size.

The number specified should generally be the aspect ratio (ratio of x-height to font size) of the first choice font-family. This means that the first choice font, when available, will appear the same size in browsers, whether or not they support font-size-adjust.